Drills frequently are used to provide cylindrical holes in metallic workpieces. As is well known, the cutting or boring action of the drill is carried out by an elongated substantially cylindrical member. One end of the drill member is securely mounted in the driving apparatus of the drill assembly which rotates the cutting member about its longitudinal axis. The opposed end of the elongated cutting member includes at least one cutting edge. A flute extends away from the cutting edge and provides a channel for removal of chips of stock that are produced as the hole is drilled.
A twist drill is one type of elongated cutting member commonly used on both wood and metallic stock. Briefly, the twist drill provides a plurality of cutting edges disposed symetrically about the perimeter of the drill and extending generally along the longitudinal axis thereof. A flute extends from each cutting edge and tyupically extends helically around the perimeter of the cutting member, hence the term twist drill. Although the cutting edges of the twist drill extend toward the longitudinal axis, they terminate a distance away from the axis.
The structural features of the twist drill have both operational advantages and disadvantages. A principle advantage is that the cutting forces are evenly distributed across the drill. Thus, in many materials, the twist drill will provide a precise hole, with each cutting edge contributing an equal amount to the cutting effort. More specifically, each cutting edge will generate chips or helical strips of stock material having a width equal to the radius of the drill, and a thickness that is a function of the feed rate of the drill into the stock divided by the number of cutting edges. A principal disadvantage of the twist drill is that the portion of the drill between the longitudinal axis and the innermost point of the cutting edges performs no cutting function. Consequently, the innermost portion of the material to be drilled is urged into the area of the cutting edges by an axial force exerted on the drill itself. In effect, the drill must be pushed into the material being drilled so that the material in the centermost portion of that hole can be urged into the path of the cutting edges.
The axial force required can be reduced by minimizing the distance between the longitudinal axis and the innermost point of each cutting edge. Of course, this distance cannot be reduced to zero because the material immediately surrounding the longitudinal axis provides the principal axial support for the twist drill. The amount of axial force can be reduced somewhat further by chamfering the central portion to encourage the movement of the stock material into the area of the cutting edges. Although these structural features minimize the axial force required to advance the twist drill, the axial forces are significant enough to make the twist drill inefficient and ineffective for stocks made of hard metallic substances, such as titanium. The exertion of these axial forces not only leads to an inefficient use of energy in a drilling operation, but also contributes to excessive wear on the twist drill itself. Thus, twist drills must be replaced frequently, thereby resulting in a substantial amount of down time for the entire drilling assembly.
A conventional "gundrill" is structurally distinct from the twist drill and, in some respects, is more desirable for drilling holes in hard metals. Briefly, the gundrill includes only one cutting edge and one flute. The single cutting edge extends from a point on the periphery of the gundrill through the longitudinal axis and to a point intermediate the opposite peripheral surface of the gundrill and the central axis thereof. The cutting edge terminates at an apex that is offset from the longitudinal axis of the gundrill so that the cutting end of the gundrill resembles an asymmetrical cone. The gundrill also includes a flute extending generally in an axial direction from the cutting edge to allow for removal of chips of stock that are cut by the gundrill. In operation, the portion of the gundrill extending from the longitudinal axis to the periphery accomplishes all of the cutting. Additionally, because the cut extends the full length of the radius, stock material does not have to be forced toward the cutting edge, and the axial force required is less than that required when using a twist drill. Due to this center cutting feature, the gundrill is more readily adapted to drilling holes in hard metals and has been widely employed in certain metal cutting applications such as drilling a hole in a gun barrel.
Although the single cutting edge and single flute structure of the gundrill provide an efficient center cut, it also results in an imbalance of forces during a drilling operation. This imbalance is particularly critical during the early phases on a drilling operation when the gundrill initially penetrates the stock. More particularly, because the gundrill has only one cutting edge, there is no symmetrical cutting surface to balance the forces exerted by the stock against the single cutting edge. The imbalance of forces with the gundrill frequently causes the gundrill to wobble which, in turn, causes a "runout" phenomena. Thus, the walls of the hole bored by the gundrill are not parallel to one another, but rather, bulge outwardly, particularly near the entry point to the hole.
To offset the inherent runout effects of a gundrill, gundrills are generally fabricated with wear pads which are adapted to bear against the sidewall surface of the drilled hole, and thereby function to guide the gundrill after its initial entry. Also, to minimize runout at the entry point of the hole, bushings frequently are employed to properly guide the gundrill into the desired location. The bushings are located on the surface of the stock and surrounding the area to be drilled. Thus, the wear pads bear against the bush and improve the alignment of the hole initial entry of the gundrill into the stock. Nevertheless, despite the use of wear pads and bushings, the imbalance of forces inherent in the gundrill design frequently causes misalignments that exceed the tolerances of many workpiece specifications. This is particularly likely to happen in workpieces made of very hard metallic substances. To attain the proper tolerances in these workpieces, a second reaming operation is often required. However, this reaming operation, like the use of bushings, is extremely time-consuming, inefficient, and costly.
In certain drilling operations, it is necessary to drill very precise holes for a short distance into a very hard material. Additionally, it often is desirable to provide a hole with a bottom that is substantially planar or flat and which is generally perpendicular to the side walls of the hole. More specifically, some design tolerances require the actual diameter of the hole to be in the range of -0.0005 inches to +0.0015 inches of the specified diameter. Also, in many specifications, the bottom surface of the hole may be non-planar provided the bottom surface includes a shoulder or ledge adjacent to and perpendicular to the side walls of the hole and extending a distance inwardly therefrom approximately equal to one half the radius of the hole. This non-planar bottom surface of the hole is acceptable only if the distance between the central portion and the peripheral shoulder portion of the hole bottom is very small.
Holes with the above cited specifications are required, for example, on the cutting head portion of an oil well drilling apparatus. In this type of apparatus, a head constructed from a very hard meterial, such as hard steel AISI 4340, has a plurality of precise shallow holes drilled therein to accept bits that will cut through rock during an oil drilling operation. Twist drills are currently used by certain tool manufacturers to make these holes. However, because of the extremely hard stock material, the twist drills have a very short life, and it often is necessary to use a reamer before achieving the desired precision hole. This, of course, is extremely expensive. It is also known to utilize gundrills for these purposes. However, as mentioned above, the gundrill is least precise during its initial penetration of the workpiece. Consequently, it is extremely difficult to obtain a precisely drilled shallow hole with a gundrill. Therefore, to achieve these design specifications with a gundrill, it is necessary to utilize a complex arrangement of bushings and also to subsequently ream the hole after the drilling operation.
Accordingly, it is an object of the subject invention to provide a drill that can precisely bore holes in a variety of materials.
It is another object of the subject invention to provide a drill that is particularly useful for drilling precise shallow holes in extremely hard material.
It is a further object of the subject invention to provide a drill that can drill a hole having a substantially flat bottom that is substantially perpendicular to the side walls of the hole, or a hole in which the bottom includes a peripheral shoulder that is substantially perpendicular to the side walls of the hole.
It is an additional object of the subject invention to provide a drill that operates so as to balance the cutting forces during a drilling operation.
It is yet another object of the subject invention to provide a drill that will reliably remove chips of stock material from the drilled hole.
It is an additional object of the subject invention to provide a drill that can drill precise holes without the use of bushings or the like.
It is still another object of the subject invention to provide a drill that can precisely drill holes without requiring a subsequent reaming operation.
It is a further object of the subject invention to provide a drill capable of drilling through hard metallic materials and which has a long life in operation.